POROSITY DETERMINATIONFROM LOGS

Most slides in this section are modified primarily from NExT PERF Short Course Notes, 1999.However, many of the NExT slides appears to have been obtained from other primarysources that are not cited. Some slides have a notes section.

Well LogSP Resistivity

OPENHOLE LOG EVALUATION

Oil sand

Gammaray

Resisitivity Porosity

Increasingradioactivity

Increasingresistivity

Increasingporosity

Shale

Shale

POROSITY DETERMINATION BY LOGGING

POROSITY LOG TYPES

3 Main Log Types

• Bulk density

• Sonic (acoustic)

• Compensated neutron

These logs do not measures porosity directly. To accurately calculate porosity, the analyst must know:•Formation lithology • Fluid in pores of sampled reservoir volume

DENSITY LOGS• Uses radioactive source to generate gamma

rays

• Gamma ray collides with electrons in formation, losing energy

• Detector measures intensity of back-scattered gamma rays, which is related to electron density of the formation

• Electron density is a measure of bulk density

DENSITY LOGS

• Bulk density, b, is dependent upon:

– Lithology

– Porosity

– Density and saturation of fluids in pores

• Saturation is fraction of pore volume occupied by a particular fluid (intensive)

GRAPI0 200

CALIXIN6 16

CALIYIN6 16

RHOBG/C32 3

DRHOG/C3-0.25 0.25

4100

4200

DENSITY LOG

Caliper

Density correction

Gamma ray Density

Formation (b)

Long spacing detector

Short spacing detector

Mud cake(mc + hmc)

Source

BULK DENSITY

fmab 1

Matrix Fluids influshed zone

•Measures electron density of a formation

•Strong function of formation bulk density

•Matrix bulk density varies with lithology

–Sandstone 2.65 g/cc

–Limestone 2.71 g/cc

–Dolomite 2.87 g/cc

POROSITY FROM DENSITY LOG

Porosity equation

xohxomff S1S

fma

bma

Fluid density equation

We usually assume the fluid density (f) is between 1.0 and 1.1. If gas is present, the actual f will be < 1.0 and the calculated porosity will be too high.

mf is the mud filtrate density, g/cc

h is the hydrocarbon density, g/cc

Sxo is the saturation of the flush/zone, decimal

DENSITY LOGS

Working equation (hydrocarbon zone)

mashshsh

hcxomfxob

V1V

S1S

b = Recorded parameter (bulk volume)

Sxo mf = Mud filtrate component

(1 - Sxo) hc = Hydrocarbon component

Vsh sh = Shale component

1 - - Vsh = Matrix component

DENSITY LOGS• If minimal shale, Vsh 0

• If hc mf f, then

b = f - (1 - ) ma

fma

bmad

d = Porosity from density log, fraction

ma = Density of formation matrix, g/cm3

b = Bulk density from log measurement, g/cm3

f = Density of fluid in rock pores, g/cm3

hc = Density of hydrocarbons in rock pores, g/cm3

mf = Density of mud filtrate, g/cm3

sh = Density of shale, g/cm3

Vsh = Volume of shale, fraction

Sxo = Mud filtrate saturation in zone invaded by mud filtrate, fraction

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

BULK DENSITY LOG

Bulk DensityLog

RHOC

1.95 2.95

NEUTRON LOG

• Logging tool emits high energy neutrons into formation

• Neutrons collide with nuclei of formation’s atoms

• Neutrons lose energy (velocity) with each collision

NEUTRON LOG

• The most energy is lost when colliding with a hydrogen atom nucleus

• Neutrons are slowed sufficiently to be captured by nuclei

• Capturing nuclei become excited and emit gamma rays

NEUTRON LOG• Depending on type of logging tool either gamma rays

or non-captured neutrons are recorded

• Log records porosity based on neutrons captured by formation

• If hydrogen is in pore space, porosity is related to the ratio of neutrons emitted to those counted as captured

• Neutron log reports porosity, calibrated assuming calcite matrix and fresh water in pores, if these assumptions are invalid we must correct the neutron porosity value

NEUTRON LOG

Theoretical equation

Nmashshsh

NhcxoNmfxoN

V1V

S1S

N = Recorded parameter

Sxo Nmf = Mud filtrate portion

(1 - Sxo) Nhc = Hydrocarbon portion

Vsh Nsh = Shale portion

(1 - - Vsh) Nhc = Matrix portion where = True porosity of rock

N = Porosity from neutron log measurement, fraction

Nma = Porosity of matrix fraction

Nhc = Porosity of formation saturated with

hydrocarbon fluid, fraction

Nmf = Porosity saturated with mud filtrate, fraction

Vsh = Volume of shale, fraction

Sxo = Mud filtrate saturation in zone invadedby mud filtrate, fraction

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

POROSITY FROM NEUTRON LOG

NeutronLog

CNLLC

0.45 -0.15

Upper transmitter

Lower transmitter

R1

R2

R3

R4

ACOUSTIC (SONIC) LOG

• Tool usually consists of one sound transmitter (above) and two receivers (below)

• Sound is generated, travels through formation

• Elapsed time between sound wave at receiver 1 vs receiver 2 is dependent upon density of medium through which the sound traveled

sec50

T0E2

E1

E3

Mud wavesRayleigh

wavesCompressional

waves

Lithology Typical Matrix TravelTime, tma, sec/ft

Sandstone 55.5Limestone 47.5Dolomite 43.5Anydridte 50.0Salt 66.7

COMMON LITHOLOGY MATRIXTRAVEL TIMES USED

ACOUSTIC (SONIC) LOG

Working equation

mashshsh

hcxomfxoL

tV1tV

tS1tSt

tL = Recorded parameter, travel time read from log

Sxo tmf = Mud filtrate portion

(1 - Sxo) thc = Hydrocarbon portion

Vsh tsh = Shale portion

(1 - - Vsh) tma = Matrix portion

ACOUSTIC (SONIC) LOG

• If Vsh = 0 and if hydrocarbon is liquid (i.e. tmf tf), then

tL = tf + (1 - ) tma

or

maf

maLs tt

tt

s = Porosity calculated from sonic log reading, fraction

tL = Travel time reading from log, microseconds/ft

tma = Travel time in matrix, microseconds/ft

tf = Travel time in fluid, microseconds/ ft

DT

USFT140 40

SPHI

%30 10

4100

4200

GR

API0 200

CALIX

IN6 16

ACOUSTIC (SONIC) LOG

Sonic travel time

Sonic porosity

Caliper

Gamma Ray

SONIC LOG

The response can be written as follows:

fmalog t1tt

maf

ma

tt

tt

log

tlog = log reading, sec/ft

tma = the matrix travel time, sec/ft

tf = the fluid travel time, sec/ft

= porosity

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SONIC LOG

SonicLog

DT

150 50us/f

EXAMPLE

Calculating Rock Porosity Using an Acoustic Log

Calculate the porosity for the following intervals. The measured travel times from the log are summarized in the following table.

At depth of 10,820’, accoustic log reads travel time of 65 s/ft.

Calculate porosity. Does this value agree with density and neutron logs?

Assume a matrix travel time, tm = 51.6 sec/ft. In addition, assume the formation is saturated with water having a tf = 189.0 sec/ft.

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SPHIss45 -15

EXAMPLE SOLUTION SONIC LOG

SPHI

FACTORS AFFECTING SONIC LOG RESPONSE

• Unconsolidated formations

• Naturally fractured formations

• Hydrocarbons (especially gas)

• Rugose salt sections

RESPONSES OF POROSITY LOGS

The three porosity logs:– Respond differently to different matrix

compositions– Respond differently to presence of gas or

light oils

Combinations of logs can: – Imply composition of matrix– Indicate the type of hydrocarbon in pores

GAS EFFECT

• Density - is too high

• Neutron - is too low

• Sonic - is not significantly affected by gas

ESTIMATING POROSITY FROM WELL LOGS

Openhole logging tools are the most common method of determining porosity:

• Less expensive than coring and may be less risk of sticking the tool in the hole

• Coring may not be practical in unconsolidated formations or in formations with high secondary porosity such as vugs or natural fractures.

If porosity measurements are very important, both coring and logging programs may be conducted so the log-based porosity calculations can be used to

calibrated to the core-based porosity measurements.

Influence Of Clay-Mineral DistributionOn Effective Porosity

Dispersed Clay• Pore-filling• Pore-lining• Pore-bridging

Clay Lamination

Structural Clay(Rock Fragments,

Rip-Up Clasts,Clay-Replaced Grains)

e

e

e

ClayMinerals

Detrital QuartzGrains

e

e

FlowUnits

Gamma RayLog

PetrophysicalData

PoreTypes

LithofaciesCore

1

2

3

4

5

CorePlugs

CapillaryPressure

vs k

GEOLOGICAL AND PETROPHYSICAL DATA USED TO DEFINE FLOW UNITS

Schematic Reservoir Layering Profilein a Carbonate Reservoir

Baffles/barriers

3150

SA -97A SA -251 SA -356 SA -71 SA -344 SA -371

SA -348 SA -346 SA -37

3200

3250

3300

3350

3100

3150

3250

3300

3250

3150

3200

3100

3150

3200

3250

3200

3250

3250

3350

3300

3150

3200

3250

3300

3100

3200

3250

3300

3350

3150

3200

3250

Flow unit

From Bastian and others